Method of sound transmission



METHOD .OF SOUND TRANSMISSION y I ,Q v l v mouw; m maso 'RANGE H400089000 4mm' .sqooof awvalucv/YJSEL v l swoniva m H Ldao y r mw my Feb. 2,llsz Filed NOV.v 16, 1945 2130 no uovmnn Ava Feb. 26, 1952 w. -lvl-.'Evimm METHOD oF' souNn TRANSMISSION 7 sheets-sheet s Filed Nov. 16,1945 Emi-ALE( Feb. 26, 1952 I w. M. EwlNG 2,587,301

METHOD oF souNn TRANSMISSION F11-ed Nov. 1e, 1945 7 sheets-sheet 5 7 8INITIAL INCLINATION DEGREES Feb. 26,"1952l w.M. EWING, 2,587,361 METHODoF SOUND TRANSMISSION v sh'eet'sv-sneet e Filed NOV. 16, 1945 INITIALINCLINATION DEGREES Patented Feb. 26, 1952 METHOD OF SOUND TRANSMISSIONWilliam M. Ewing, Woods Hole, Mass., assigner, by mesne assignments, tothe United States of America as represented by the Secretary of the NavyApplication November 16, 1945, Serial N0. 629,048

6 Claims.

This invention relates to methods of sound transmission and especiallyto long-range transmission of sound through a medium, such as sea water.More particularly, the invention is conc erned with a method oftransmitting sound signals in which special phenomena encountered inlocean depths are made use of to extend the distances over which audiblesound signals may be transmitted.

In one specific aspect the invention deals with a method ofcommunication of specialized character adapted for use by submarines andother vessels, in connection with secret military operations, to enablethe personnel of such vessels to send back information from a vantagepoint at which radio transmitters are unsatisfactory.

to occur along' rays which are subject to refraction by variations insound velocity. In most areas of the oceans, the velocity of soundvaries with depth and temperature, and is less at some intermediatedepth than it is either at the surface or at the ocean bottom. Theseoceanographic conditions produce a "sound channel having at itshorizontal axis a level along which the velocity of sound reaches itsminimum value. Essentially, the sound channel provides for a long-rangetransmission of sound between a sound source and a receiver Where bothtypes of sonic equipment are located at a level approximatelycorresponding to the axis of minimum velocity.

Considering the bathic aspects of the sound channel in greater detail,it is found that in going from a point just below the surface of seaWater to increasingly greater depths, there occurs a decrease intemperature and a corresponding decrease in sound velocity. This effectcontinues through depths ranging from' 675 to 700 fathoms. In thisregion, a minimum sound velocity level is reached beyond which littlefurther temperature change is observed. Below the minimumlvelocitylevel, increasing pressure, with increasing depth, causes the velocityof sound to increase gradually at a relatively slower rate compared toits rate of decrease, and ultimately at very great depths of 2000 to3000 fathoms to regain and exceed the initial velocity present justbelovvjlthe ocean surface.

Therefore, from a refraction point of view, variation of velocity withtemperature may be regarded as forming an upper side or roof for thesound channel; variation of sound velocity with pressure may be regardedas forming a lower side or floor for the sound channel. It should alsobe observed that the so-called roof or upper I.'Ihe propagation of soundin sea water is known side of the sound channel is located away from theaxis of the channel a distance much less than the distance between theaxis of the lower side or floor of the channel.

I have discovered that a bathic sound channel may be utilized as arefracting medium to achieve two important objectives. One is theseparation of a source of sound energy into a series of successivelyoccurring sound impulses which are characterized by progressivelygreater intensity, leading up to an abrupt and highly identifiableendpoint. A second objective is to prevent loss of sound energy due toreflection of sound rays at the surface or bottom of the ocean, and thusr to provide for long-range sound transmission.

Since the minimum sound velocity axis occurs at a relatively great depth(approximately 700 fathoms), there is provided above and below thislevel a distance through which a limited number of sound rays may becompletely refracted by the sound channel and may be turned back withoutever reaching the surface or bottom of the ocean, thereby to furnishcontinuous paths along which it is believed that sound impulses maytraverse great distances. Only a limited number of rays whose angles ofinclination to a horizontal axis are such as to permit completerefraction by the roof of the sound channel are available for thispurpose. In Fig. 1 of the drawings a typical sound channel together witha series of refracted sound rays has been diagrammatically indicated,and extending across the diagram in a horizontal manner is a heavy lineintended to further indicate the approximate axis of the sound channel.

It is suggested that sound energy instead of being absorbed in passingthrough a body such as sea water, as is commonly believed, is largelydissipated as a result of reflection at the surface and bottom of theocean. When reection of the sound rays is prevented as a result of therays being refracted to form continuous reversely bent paths whichextend in a generally horizontal direction, there occurs little loss ofenergy. This feature persists to a useful extent even though thereoccurs considerable deviation. Sound signals occurring along suchreversely bent rays may in this way be transmitted over distances ofseveral thousand miles. This explanation of sound energy dissipation isoffered as a matter of opinion only and is not to be regarded aslimiting the invention to it in any sense.

Essentially, therefore, the invention includes a method of soundtransmission in which a sound is generated in a medium of varyingrefractive power at a minimum sound velocity level. At

a. distant point in the medium and at the same sound velocity level, aseries of successively occurring sound impulses emanating from thegenerated sound are observed for the purposes of identifying andlocating the generated sound and receiving communications. Best resultsmay be obtained by generating and observing sound signals at regionsalong the minimum sound velocity level extending upwardly for a distanceapproximately half-way between the minimum velocity level and the roofor surface of the sea. and extending downwardly a distance approximatelyhalf-way between the minimum sound velocity level and the floor or oceanbottom. Beyond these regions, results are progressively poorer.

An important feature of the invention is a. method of transmitting soundthrough sea water over relatively great distances and in a. manner suchthat the transmitted sound signal is distinguishable from various othersounds which may occur in the ocean, to provide a. novel communicationsystem.

Another important feature of the invention is a method of soundtransmission characterized by the fact that a signal may be generatedand propagated without interference or' the type com` monly referred toas radio jamming Another feature of this invention is that by notingparticularly the time interval between reception of signals received attwo widely separated stations, a geographical line of position may bededuced which is known to pass through the location of the sound source.

lAnother feature of this invention is that by noting particularly thetime of reception of signals at more than two widely separated stations,geographical lines of position may be established from each pair ofstations, the intersection of these lines denotes the exact location ofthe sound source.

Another feature of this invention is the combination with a radio signalemitted at the sound source in synchronism or in known time relationwith the acoustical signal. By noting particularly the time intervalbetween reception of these two signals at any receiving station, thedistance of the sound source from this station may be computed.

Another feature is a method of sound transmission in which the durationof the signal received and its intensity provide a measure of thedistance which the signal has traveled.

Various other features will appear from the following description.

In the accompanying drawings:

Fig. 1 illustrates a chart diagrammatically indicating propagation ofsound along sound rays in sea water;

Fig. 2 is a velocity depth chart diagrammatically indicating temperatureand pressure conditions in sea water;

Fig. 3 is a diagrammatic view illustratingv the method of the invention;

Fig. 4 is a block diagram illustrating equipment employed in carryingout the invention;

Fig. 5 is a view in cross section indicating a. hydrophone;

Fig. 6 illustrates another type of hydrophone;

Fig. 7 is a chart illustrative of the use of stations in carrying outthe method of the invention;

Fig. 8 is another chart carrying data utilized in connection with theinvention;

Fig. 9 is still another chart of the character referred to; and

Fig. 10 is a table of data utilized ln connection with the charts shownin Figs. 8 and 9.

The drawings and description pertaining thereto deal with a preferredembodiment of the invention relating to sound transmission in sea water.However, it is intended that the invention may be practiced in otherforms as for example one in which sound is transmitted through othermedia characterized by varyingrefractive power such as for example theatmosphere and other bodies.

In carrying into eiect a preferred embodiment of the method of theinvention, I rst determine the minimum sound velocity level or axis foran ocean area. over which sound is desired to be transmitted. Such adetermination may be obtained by reference to known oceanographicinformation. Inspection of the published oceanographic data relating totemperature, salinity and pressure, shows for example that over much ofthe total area of the oceans, there is a permanent sound channel at adepth of from 675 to 700 fathoms, as illustrated in Fig. 1.

Attention is directed to Fig. 2 in which ls illustrated a downwardlyextending curve prepared from published data and representing a changein sound velocity occurring with a change in depth for a large part ofthe North Atlantic Ocean. It will be observed that the upper portion ofthe curve extends downwardly and from right to left as viewed in Fig. 2,indicating a decrease in sound velocity which is caused by a temperaturedrop throughout that particular region.

At a depth of approximately 675 fathoms, the curve starts to change itsdirection and to move downwardly and from left to right as viewed inFig. 2. At the point of change, therefore. the velocity of sound is at aminimum and at greater depths gradually increases in value. This iscaused by absence of further change in temperature and increasingpressure with depth.

The minimum sound velocity value measured at the point of change ofdirection of the curve is about 4880 feet per second. The greatestvelocity occurring above and below this depth is about 5010 feet persecond which occurs at the surface and at a level of 2030 fathomsrespectively. The sound channel in this case may be described asextending from the surface of the ocean to a depth of 2030 fathoms, withits horizontal axis at a level of 675 fathoms and having a velocitydiierential of about feet per second.

The velocity depth curve of Fig. 2, although actually representing therelation between velocity of sound propagation and depth beneath thesurface at one place, may as already noted be considered as typical fora large area-in fact for the greater part of the whole NorthAtlantic-due tn the fact that only slight changes in sound velocityoccur over very large horizontal distances.

At some desired point A (Fig. 1) in an ocean area having sound velocitycharacteristics, of which the curve of Fig. 2 is representative, soundgenerating means are put into the ocean. The sound generating unit issuspended at a depth corresponding to, or in the general region of theminimum sound velocity level.

I have found that one convenient method of generating sound atsubstantial depths is to employ explosives, such as a depth chargeincluding an electric blasting cap and a bomb consisting of TNTcontained in rubber bags, which may be detonated in depths up to 3000fathoms. The bomb may be controlled by a ring mechanism which isreleased from a surface ship or underwater vessel. The firing mechanism,together with the bomb, is caused to sink rapidly to a depthcorresponding to a minimum sound velocity level, and then to slow downand ilre the bomb. A plurality of bombs detonated successively may alsoserve to provide a means of communication.

Sound receiving equipment is also put into the ocean at some distantpoint as B (Fig. 1). at a depth generally corresponding to the soundaxis level at which point A occurs. The sound receiving equipmentincludes means for detecting sound impulses and means for converting thesound impulses into `ectrical impulses. One preferred arrangement y*hasbeen indicated in Figs. 4, 5 and 6. Fig. 4` is a block diagram, more orless self-explanatory, illustrating amplifying means, radio receivermeans, and oscillograph means which are employed in a manner familiar tothose skilled in the art, to provide audible and visual indications ofsignals picked up by a hydrophone at the sound axis.

Considering in detail the hydrophone illustrated in Figs. 5. I indictaesa rubber casing in which is supported a body of oil I2. Immersed in thebody of oil is a transformer I4 having connected at its lower end apiezoelectric crystal I6. The crystal is so made that minute acousticpressures are mechanically transformed to produce opposite charges ondifferent faces of the crystal. A modified arrangement in which thehydrophone is attached directly to amplifying means I8 in a casing 20has been indicated in Fig. 6. Various other conventional arrangementsmay be utilized.

In Fig. 3 I have indicated diagrammatically a number of suitable ways ofutilizing sound generating and sound receiving equipment of which theabove described apparatus is intended to be illustrative. With referenceto the sound generating means, I have shown a ship S at the surface ofthe ocean, from which a bomb 22 may be dropped and in the manner alreadynoted caused to explode at or near the minimum sound velocity axis 24.This operation may be carried out in other ways, as for example bydropping a bomb 26 from an airplane P; or by discharging a. bomb 28 froma submerged submarine T; or by dropping a bomb 30 from a life raft R.Similarly in receiving the signals from detonated bombs a hydrophone 32located at or near the axis of the sound channel may be suspended from avessel 34; or ya hydrophone 36 may be suspended from an automatic radiobuoy 38 for transmitting signals to shore or ship station; or ahydrophone may be connected directly to a short station 40.

Fig. 1 illustrates refraction of sound rays as transmitted in a typicalbathic sound channel. The sound generating means provides anondirectional sound source at the point A located on the axis of thesound channel. Sound rays diverge in all directions from this source;however, only those rays diverging in significant directions are shownon the diagram. On each ray is denoted the number of degrees in theangle formed by that ray with respect to the horizontal axis. This angleis referred to as the angle of initial inclination for the ray. It isimportant to note that for the depth noted of 675 fahoms along which theapproximate axis of minimum sound velocity occurs, there will be alimited number of rays which will be subject to complete refraction.Such a ray is repret 6 sented by one having an angle of initialinclination 'of 12.19, together with a group of rays have ingprogressively smaller angles of inclination as shown.

Therefore it may be readily seen that any ray with initial inclinationof l12.19 above the hori zontal and below the horizontal will berefracted back and forth between the upper and lower parts of the soundchannel in such a way that it can travel any desired distance withoutthe necessity of undergoing reflections at either the surface or thebottom of the ocean. A ray having an angle value in excess of 12.19 willnot be completely retracted and will be subject to reection withconsequent dissipation of energy. Such a ray is illustrated by the raynoted with the angle value of 15.19 in Fig. 1.

Another important aspect of the invention resides in the fact that theaverage speed of forward progress is greatest for those rays which, inFig. 1, have the largest initial inclinations to the horizontal soundaxis. Although the actual path length is greater for these rays than forthose with small'angles. this added distance is more than cancelled bythe fact that much of the distance is traversed in regions where thevelocity of propagation is much greater than at intermediate regions.These relations are indicated in Figs. 8. 9 and 10.

The relation between mean horizontal velocity and the initialinclination of the ray, for the typical sound channel considered in Fig.1, is shown in Fig. 9. It may be seen from an inspec tion of Fig. 1 thatthe largest initial angle which a ray may have and still remain in thesound channel is 12.19, and that the horizontal distance traversed inone cycle of this ray is about 85,500 yards, or about 42 miles (Fig. 8)

Now, for a hydrophone on the axis of the sound channel at a distance ofsay 1050 miles, this ray will reach the hydrophone after making 25cycles (1050 miles divided by 42) in its pattern. Moreover, the soundwhich travels along this 12.19 ray will be the i'lrst sound from thegiven source `to reach the hydrophone, for it has the greatest possiblemean horizontal velocity of 4936.0 ft./sec., as shown in Fig. 9. The raywhich makes 26 cycles in travelling between these same two points willmake yards per cycle. Fig. 10 is a table giving in celumn B the numberof yards per cycle for the rays making various numbers of cycles between25 and 150, for the example in question. namely, where shot and receiverare about 1050 miles apart.

-The initial inclination. 0 in column C, Fig. 10, may be readapproximately for each value of the horizontal distance travelled percycle from the ray diagram of Fig. l, but more accurately from Fig. 8.The inclinations obtained are shown in Fig. 10 column C. For each valueof the initial incllnation in Fig. 10, the corresponding value of themean horizontal velocity V is read from Fig. 9 and entered in column Dof Fig.`10. Each of these values of mean horizontal velocity may be usedto nd the travel time for the corresponding path by simple division intothe distance assumed for this example, namely 25 85,500 yards. or about1050 miles. These travel times are given in column E of Fig. 10.

It may be seen from Fig. 10 that the ray for the rst sound to arrivemakes 25 cycles, correkhas a travel .time of the total ydin' ,f ginningof t that'one'couldexpectthefsuccessive arrivals tor beA recognizedy as.such by merely listening to thenLvr But it is seen that' theinterval'betwen y y which comprisesgenerating sound insa fluid me` l f fspending to an initi y y kr1299.13 seconds, while that, forv thelastoney has vapri initial inclination' of avel time of y13l4.03seconds.

.f 'this distance would beabout 15 seconds.' Fur,-y

' thermore, the f timefr interval betweenr successive y 'arrivals isvapproximately 0.5 second at the be-i l he signal, which is sufficiently,long

' successive arrivals becomes vless and. lessi, so that;

This `considera f clusion thatthe time arrivals; becomes shorter ankreceivedfalong l y l y "creasesfthroughout the duration fofA thesignal.

j yCalculations i vwere idealized to have a; s rabove-the axis andasingle one :below it, then thty intensity ,of signal received 'alongyagiven ray i vwould ybe approximately fthe sainey for all rays.

` ble to make. the following deductions;r Thevapproximate duration ofthe seconds at this distance. k,'lfliiere occurs a gradual increase invthe average intensity of sound from; f

' signal vtoj its'end lfl'lfiere l y y 'athebeginning of the t n ns inintensity yduring the arey audible ncituatio early part of signal; ,w

- toward the end olfv the Isignal these arrivals will y y f undoubtedlymergei together 'and' becomey y l solvable. either by.' yearoiiby theoscillo'graph lre`vr cording.

ingle velocity lgradient that Y' the average in l 'that isf vrthe energyreceived per yunit of ,tinfieawhich is'due to t he roo'lllbrld actiongoithel sounds k'mais for the vInstance noted; n becomes: passiy ithfluctuations increasing in tempo during the they becomeindistinguishable toward the end. The end of the signal consists of anabrupt drop in intensity from maximum value to zero. A signal having theabove features is so characteristic that it is readily distinguishablefrom sounds due to explosions at other depths. The signal consistspredominantly of low frequency sound.

The method of the invention may be utilized as a means of communicationin several ways. One application will be readily seen to be conveyinginformation by means of a code based on observation of time intervalsbetween successive signals in the manner already discussed, andindicated diagrammatically in Figs. 1, 2 and 3. Another application ofthe invention consists in determining the geographical position of asound source. Fig. '7 is intended to better-indicate this operation. Inthis figure three stations are illustrated, including for instance astation at Midway island, another station at Saipan island, and a thirdstation at the Aleutian Islands. For each station, a series of distancecurves are drawn based on known travel times of sound in sea water. Eachof the three stations is equipped with suitable receiving equipmentlocated at proper sound axis levels and connected to the station. Theparticular time of arrival of a sound signal at each station is notedand from this an indication of the approximate distance is obtained andplotted on the proper distance curve. The data thus obtained by onestation are communicated by radio or other means to a second station.The operator there then plots the point of intersection of twogeographical al inclination 'of 12.19,iand

show that if' the sound channel n n y t shorter, ,indicates y n tensityofl sound received-ie i i v ized :by a ygradiially increasing .intensltmvandan y, f abruptending. 1

signal 'to' the vdegree thatr ation of received rsignal at l y ifio'usvother arrangements may along' the f' same ysound .velocity levelr linesof position and thuSf geographical sound source., to yuse kthe soundsignal in Yalol sorted to, in keepingwith therispirit of the invenrenceyiof successive sound impulsesl rfrom the generated sound.y d v, f1scundftransmission i whichcomprises generating sound'in ftheocean; f atits minimum vsour'id velocityilevel. and observe v ing 'atajdistant-'point along thesamesoundve-- if xes the approximate radiosignal and notingr the i tween'the'reception'o 'the different signals.Var-v v f y lVilhile' I lhaine showin a preferred: embodiment 'of myinvention, itk should bej `1`.` Improved method; 0.15v soundftransmission mum at its minimum sound 'veracityl level, and k; L

be employed. f v f understoodthatmj f l schangesandt modifications' maybeL ire-,f l

bbservmg at amener :pome m the, said medium,

emanating locity level the occurrence'ofaninterrupted ser-1 tion,takentogetherwithfthecone i v i interval betweenl successive; quence; ofsound. impulses; emana;tingy from vthe f generated sound.- said impulsesbeing charactlerf v f i pulses remanating from; the explosion. r f

ed communication i method i g f i all possible individual rays, 'in-n'which comprises detonating an explosive'in the .f y ','oceanat itsminimum soundzvelocity level and lobserving at another` point intheoceanalong the' f g f @same sound velocitylevel refracted'sound im@yf,

4. 'That improvedmethodwhich comprises det-,g f g l f vonatinganexplosiveiin the ocean at its minimum i l y explcsion'ink a series'sound' velocity level to separate Ithesound of; the f f v of 'Soundrays, reordins jat i l ff l another point on the ocean along the 'samesoundy f l velocity levelasuccessfioniof refractedsound im-v-v y pulsesemanating from the explosion, 'and'then' measuring the change in timeintervals occurring between successive sound impulses to determine thelocation of said generated sound.

5. Long-range transmission of sound under water which comprisesgenerating a source of sound at a depth at which refracted sound raysemanate from the said source of sound and converge repeatedly along asound channel, and observing a succession of sound impulses arriving ata point in the channel remotely located with respect to the location ofthe sound source.

6. Method of communication which comprises generating sound in a fluidmedium at its minimum sound velocity level to separate the sound into aseries of characteristic sound impulses, and then observing from a pointalong the sound velocity level the time interval between reception ofsignals at two widely separated stations to arrive at a geographicalline of position which passes through the sound source.

WILLIAM M. EWING.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 913,528 Marriott Feb. 23, 19091,149,976 Furber Aug. 10, 1915 1,240,328 Fessenden Sept. 18, 19171,706,066 Karcher Mar. 19, 1929

